Biomimetic and variable stiffness ankle system and related methods

ABSTRACT

A cam system for an assistive device and related methods are disclosed. The cam system may comprise a cam profile and a cam follower. The cam profile has a curved outer edge comprising a concave portion. The cam follower is positioned within the concave portion when the assistive device is in an equilibrium position. The assistive device may further comprise a spring that deflects in response to a force applied by the cam system. The assistive device may have a sliding element to adjust the stiffness of the spring in deflection. The assistive device may take the form of a prosthesis or an orthosis.

RELATED APPLICATIONS

This application claims benefit to U.S. Provisional Patent ApplicationNo. 62/403,597 filed on Oct. 3, 2016 and U.S. Provisional PatentApplication No. 62/412,583 filed on Oct. 25, 2016, both of which areincorporated by reference.

BACKGROUND

Ankle-foot prostheses are used by individuals who do not have an ankle.Many commercially available prosthetic ankle-feet behave much likesprings, storing and returning energy to the wearer. However, thetorque-angle relationships of these prostheses do not appropriatelyfollow the torque-angle relationship (or “quasi-stiffness” curve) ofable-bodied walking.

Some ankle-foot prostheses, such as a Solid Ankle-Cushion Heel (SACH)Foot, are stiffer during stance phase than a human ankle would be in asimilar phase. Other ankle-foot prostheses, such as Energy Storage andReturn (ESR) feet, are less stiff, but do not appropriately mimic thelow quasi-stiffness during early stance, and the rapidly increasingquasi-stiffness before heel-off. This may in part be due to thedifficulty of designing devices with nonlinear stiffnesscharacteristics. As a result of the relatively high stiffness duringmid-stance of such prostheses, the individual's tibial progression ishindered, which may slow the self-selected walking speed. The relativelyhigh stiffness of such prostheses during controlled plantarflexionimmediately following heel strike may also prolong heel-only contact,reducing stability and inhibiting tibial progression.

ESR feet are also unable to modulate their mechanical properties toimprove function during other mobility tasks. For instance, during quietstanding, transtibial amputees exhibit increased postural sway.Additionally, ESR feet exhibit significantly reduced range of motionduring stair traversal, likely due to their high stiffness makingforward progression of the center of mass more difficult. In addition,they are unable to capture the change in potential energy of the centerof mass that results from tibial progression. For these tasks, amputeesdevelop abnormal compensatory gait patterns which can lead to a widerange of issues, such as socket pain, back pain, and joint diseases.

Some ankle prostheses vary damping characteristics. Changing anklemechanics with damping variation may be counterproductive, since energyis being removed from the ankle joint, which provides a majority of themechanical energy needed for forward propulsion in able-bodied gait.

It would be useful for an ankle-foot prosthesis to exhibit appropriatebiomechanics during various phases of gait, such as walking, stairtraversal, or quiet standing. Likewise, it would be useful for otherassistive devices, such as orthoses, to help patients exhibitingsimilarly appropriate biomechanics using their sound limbs.

BRIEF SUMMARY

A cam system for an assistive device is disclosed. The system maycomprise a cam profile and a cam follower. The cam profile may have acurved outer edge comprising a concave portion. The cam follower may bepositioned within the concave portion when the assistive device is in anequilibrium position.

The cam profile may be positioned to rotate about a joint of theassistive device in response to a force applied to the assistive deviceduring a stance phase of gait.

The cam follower may be coupled to a leaf spring of the assistivedevice. The cam follower may roll along the curved outer edge of the camprofile from dorsiflexion to plantarflexion of the assistive device.

The cam profile may have a convex portion on which the cam follower ispositioned during plantarflexion of the assistive device. The camprofile may have a convex portion on which the cam follower ispositioned during dorsiflexion of the assistive device.

An assistive device comprising a cam system is disclosed. The assistivedevice may comprise a leaf spring. The cam system may be positioned atan end of the leaf spring such that a force applied to the cam systemcauses deflection of the leaf spring. The assistive device may comprisea sliding element to adjust the stiffness of the leaf spring upondeflection. The position of the sliding element may be adjustable alonga surface of the leaf spring.

The sliding element may be positioned on a motor-powered screw thatadjusts the position of the sliding element on the surface of the leafspring. The position of the sliding element along the surface of theleaf spring may be manually adjustable. The assistive device maycomprise a Bowden cable attached to the sliding element to manuallyadjust the position of the sliding element.

The assistive device may be an ankle prosthesis or an orthosis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following descriptions of the drawings and example assistive devicespresent certain aspects, wherein like reference numerals across theseveral views refer to identical or equivalent features, and wherein:

FIG. 1 displays a series of able-bodied ankle quasi-stiffness curvesduring stance phase, for level-ground walking, stair descent, andstanding.

FIG. 2A displays a front view of an example ankle prosthesis.

FIG. 2B displays a side view of an example ankle prosthesis.

FIG. 2C displays another side view of the example ankle prosthesis.

FIG. 3A displays a first diagram of a mathematical description of thecam profile and the cam follower.

FIG. 3B displays a second diagram of a mathematical description of thecam profile and the cam follower.

FIG. 4A displays an illustration of how moving a simple support towardsa load increases spring stiffness.

FIG. 4B displays a graph indicating experimental results oftranslational leaf spring stiffness as a function of the sliderposition.

FIG. 5 displays a graph that plots slider position as a percentageagainst time, depicting step responses from standing stiffness to otherstiffnesses in an example of an ankle prosthesis.

FIG. 6 displays a graph that plots a primary desired quasi-stiffnesscurve for walking, wherein the shaded region reflects the range ofpossible stiffnesses.

FIG. 7 displays a series of plots that experimentally characterizestiffness profiles of an exemplary ankle prosthesis at differentstiffness profiles.

FIG. 8 displays a representational diagram for a method of deriving acam profile.

FIG. 9 displays a pictoral representation of a system for controlling anankle prosthesis.

FIG. 10 displays a side view of an example passive ankle prosthesis.

FIG. 11 displays a side view of an example orthosis.

FIG. 12 displays a view of an exemplary cam profile with a curved outeredge.

DETAILED DESCRIPTION

“Dorsiflexion” is the flexion of a human ankle or an assistive devicesuch that the angle between the foot and shin decreases. Indorsiflexion, the toes on a human foot point upwards towards the shin. A“dorsiflexion position” is a position of the ankle or assistive devicewhen it is in dorsiflexion.

“Plantarflexion” is the extension of a human ankle or an assistivedevice such that the angle between the foot and the shin increases. Inplantarflexion, the toes on a human foot point away from the shin. A“plantarflexion position” is a position of the ankle or assistive devicewhen it is in plantarflexion.

An “equilibrium position” refers to the position of an assistive devicewhen the assistive device is between a dorsiflexion position and aplantarflexion position.

“Stance phase” of gait refers to the phase of gait when a foot orprosthesis is touching the ground during gait.

“Swing phase” of gait refers to the phase of gait when the foot orprosthesis has left the ground.

Example ankle prostheses disclosed herein are designed to closely matchhuman biomechanics during gait. The ankle prosthesis may comprise acam-based transmission and a stiffness modulation unit. The cam-basedtransmission may comprise a cam profile, a cam follower, and a spring.The stiffness modulation unit can be adjustable to adjust the stiffnessof the spring in the cam-based transmission. The cam profile may bemachined to create a specific torque-angle relationship depending on theadjustment of the stiffness modulation unit. As used herein, the phrase“primary stiffness modulation unit position” refers to a pre-determinedposition of the stiffness modulation unit, and the “primary stiffnesscurve” refers to a specific torque-angle relationship provided by theankle prosthesis when the stiffness modulation unit is at the primarystiffness modulation unit position.

A quasi-passive ankle-foot prosthesis is disclosed. The prosthesisemploys a customizable torque-angle profile for gait. The anklestiffness can be varied continuously. The ankle-foot prosthesis maycomprise a cam-based transmission, in which rotation of the ankle jointin the prosthesis causes a cam follower to deflect a leaf spring, and arepositionable sliding support beneath the leaf spring that can changethe beam's stiffness. In a preferred example, the mechanism islightweight and fits into an anthropomorphic physical envelope. Anklestiffness may be adjusted during the swing phase of locomotion, as wellas between ambulation modes such as between standing and stair traverse.The cam profile may be designed on the basis of a mathematical model tocreate a desired stiffness profile.

Quasi-passive ankle-foot prostheses are disclosed. An example prosthesisemploys a customizable torque-angle profile for gait. The anklestiffness can be varied continuously. The ankle-foot prosthesis maycomprise a cam-based transmission, in which rotation of the ankle jointin the prosthesis causes a cam follower to deflect a leaf spring, and arepositionable sliding support beneath the leaf spring that can changethe beam's stiffness. In a preferred example, the mechanism islightweight and fits into an anthropomorphic physical envelope. Anklestiffness may be adjusted during the swing phase of locomotion, andbetween ambulation modes. The cam profile may be designed on the basisof a mathematical model to create a desired stiffness profile.

An example passive ankle-foot prosthesis employs a customizabletorque-angle profile for gait. The ankle stiffness can be variedcontinuously. The ankle-foot prosthesis may comprise a cam-basedtransmission, in which rotation of the ankle joint in the prosthesiscauses a cam follower to deflect a leaf spring, and a repositionablesliding support beneath the leaf spring that can change the beam'sstiffness. In a preferred example, the mechanism is lightweight and fitsinto an anthropomorphic physical envelope. Ankle stiffness may beadjusted during the swing phase of locomotion, and between ambulationmodes. The cam profile may be designed on the basis of a mathematicalmodel to create a desired stiffness profile.

FIG. 1 displays a series of able-bodied ankle quasi-stiffness curvesduring stance phase, for level-ground walking, stair descent, andstanding. The bolded portions of each curve indicate controlleddorsiflexion for walking, and heal strike to toe-off for stair descent.Standing stiffness is an estimate of the purely passive ankle stiffnessrequired to explain sway dynamics in quiet standing. Positive torque isplantarflexive, positive angle is dorsiflexion.

FIGS. 2A and 2B display a front view and a side view, respectively, ofan exemplary ankle prosthesis 201. The ankle prosthesis 201 comprises acam-based transmission, in which a cam profile 252 at the ankle joint205 compresses a leaf spring 210. In the example shown in FIGS. 2A and2B, the stiffness modulation unit, and a slider 220, which activelymodifies the support conditions of the leaf spring 210 to affectstiffness. For ambulation modes other than walking, the slider 220 canbe repositioned along the length of the lead screw 232 to modify theshape of the torque-angle relationship to be more or less stiff,deviating from the primary stiffness curve. When the slider 220 ispositioned closer to the attachment block 266, the stiffness of the leafspring 210 increases. When the slider 220 is positioned further from theattachment block 266, the stiffness of the leaf spring 210 decreases.The slider 220 may comprise a roller or other bearing system to reducefriction between the slider 220 and the leaf spring 210.

A user wearing the ankle prosthesis 201 causes the ankle prosthesis 201to rotate about the ankle joint 205 by placing a load on the ankleprosthesis 201—for example, when the user is walking and places the heel265 into contact with the ground, the user transfers his or her weightonto the ankle prosthesis 201. As the user applies such a force, theupper frame 263 begins to rotate about the ankle joint 205 with respectto the frame 262. The cam profile 252 is rigidly affixed to the upperframe 263, such that the cam profile 252 likewise rotates around theankle joint 205. As the cam profile 252 rotates, the cam follower 254rolls along the cam profile 252, both during dorsiflexion andplantarflexion of the ankle prosthesis 201. Rotation of the prosthesis201 at the ankle joint 205 causes downward deflection of the camfollower 254, which causes deflection of the leaf spring 210. Thisdeflection causes a restoring torque at the ankle joint 205.

FIG. 2C displays a side view of the prosthesis 201 in a dorsiflexionposition, while the leaf spring 210 is under deflection. A force isapplied to the upper portion of the prosthesis 201 comprising adaptor256, which causes the cam profile 252 to rotate about the ankle joint205 205. Rotation of the cam profile 252 applies a downward force on theproximal end of the leaf spring 210, causing the deflection of the leafspring 210 shown in FIG. 2C.

Cam Design.

The cam profile 252 governs the relationship between angle of the ankleprosthesis 201 at the ankle joint 205, and the associated deflection ofthe leaf spring 210 (thus, governing the ankle torque-angle). The camprofile 252 can be machined to create a specific torque-anglerelationship for the ankle prosthesis 201 based on the primary sliderposition. Exemplary mathematics governing the relationship between camprofile 252 and the ankle torque-angle curve are presented below.

In an example, the cam follower 254 has a diameter of 19 mm and adynamic load rating of 4 kN (Misumi USA, Schaumburg Ill.). The camprofile 252 may be machined from an appropriate material, such as toolsteel, which has a hardness and resistance to deformation. The camprofile 252 may be hardened to approximately 60 Rockwell C to preventdeformation from the high loads. The shape of the cam profile 252corresponds to the primary stiffness curve, which mimics the able-bodiedquasi-stiffness curve.

The cam profile 252 can be machined so that during a specific ambulationmode, such as walking, the cam profile 252 creates specific torque-anglerelationship (the “primary stiffness curve”) at a specified position ofthe slider 220 (the “primary slider position”). For other ambulationmodes, the slider 220 can be repositioned to modify the shape of thetorque-angle relationship to be more or less stiff, deviating from theprimary stiffness curve, for different ambulation modes, such asstanding or stair descent.

Leaf Spring.

To vary stiffness, the support condition of the leaf spring 210 can beactively varied continuously throughout the length of the leaf spring210. The leaf spring 210 may be coupled to a clamp support 270anteriorly towards the toe 274, and a slider 220 in the mid-foot region.A lead screw 232 positions the slider 220 along a range of motion sothat the position of the slider 220 is adjustable along the bottomsurface of the leaf spring 210, as shown in FIG. 2B. In an example, thelead screw may be a 6.35 mm diameter, non-backdriveable lead screwcapable of dynamic loads up to 700 N, with 1.27 mm lead (NookIndustries, Cleveland, Ohio, USA), which is able to position the slider220 along an 85 mm range of motion. A DC motor 230 powers the lead screw232. In an example, the DC motor 230 is a 10 Watt brushed DC Motor(Maxon Motors, Switzerland) with a 3.9:1 planetary gearhead and capableof 25 mNm intermittently. The DC motor 230 also may have an integrated1024 counts-per-revolution incremental encoder 260, which may be usedfor positioning of the DC motor 230.

The leaf spring 210 may be constructed from unilateral fiberglass(Gordon Composites, CO, USA). Fiberglass can be a useful material inthis application because of its high energy-storing capacity, making itup to eight times lighter and smaller than an equivalent spring made ofsteel. The leaf spring 210 may be shaped for roughly equal stresses onthe outer strands at the primary slider position. In a preferredexample, the height of the leaf spring 210 varies, as shown in FIG. 2B,resulting in lower shear stresses and making it less likely fordelamination to occur. In other examples, the width of the leaf spring210 may be varied rather than or in addition to varying the height ofthe leaf spring 210.

The rotational stiffness of the leaf spring 210 can be derived fromexperimental results of translational spring stiffness, shown in FIG. 4,and the effective moment arm. To predict ankle stiffness curves atdifferent leaf spring stiffnesses, which are generated by differentslider positions, a forward model may be employed that reverses throughthe presented equations. The translational stiffness of the spring,position of the virtual spring pivot, and spring preload are adjusted tosimulate movement of the slider.

The translational stiffness of the leaf spring as a function of sliderposition may be found using a material testing system, such as Sintech20G (Singapore, Singapore). Over an order of magnitude of translationalstiffness modulation may be possible within the slider's range ofmotion, for example, 0.17-2.8 kN/mm.

Mechatronics.

FIG. 9 displays a representation of an exemplary mechatronic system usedto power and control the electrical components of the prosthesis 201.The low-level positioning of the DC motor 230 may be performed by amotor controller 412, such as an EPOS2 motor controller (Maxon Motors,Switzerland). A computer 414, which may be a single board computer suchas a Linux computer (Raspberry Pi Zero, Raspberry Pi Foundation,Cambridge, UK) sends desired position to the motor controller 412, forinstance by a step-direction method. In a step-direction method, eachpulse is scaled to move the slider 220 by a predetermined amount, suchas 1 mm, which provides sufficient resolution for stiffness modulation.For example, the motor controller 412 may cause the motor encoder 260 todrive the DC motor 230 an appropriate amount to turn the lead screw 232,which moves the slider 220 the 1 mm length in the intended directionalong the lead screw 232. The computer 414 also can read ankle position,measured by an encoder 258, such as a 14-bit absolute encoder (AS5048AAMS, Premstaetten, Austria), via an interface 416 such as a SerialPeripheral Interface. Stiffness modulation may be prevented when theankle joint 205 is outside of a predetermined limit, such as ±1° ofneutral, as measured by the encoder 258. When the ankle joint 205 isoutside such a range, that indicates that the spring 210 is compressedbeyond the preload, and movement of the slider 220 is not possible withthe available torque provided by the DC motor 230. The computer 414 cancommunicates with a host computer 424 over known communicationprotocols, such as Wi-Fi. A battery 426, such as a 14.8 V, 430 mA-hourLithium-Polymer battery (Venom, Rashdrum, ID, USA) powers the motorcontroller 412, the computer 414, any associated electronics, and the DCmotor 230. The computer 414, motor controller 412, and battery 426 maybe housed in an electronics box 405, which can be mounted to theprosthesis socket, making the design entirely mobile for non-tetheredoperation.

To alter mechanics during swing phase, the slider 220 needs toreposition quickly. The reference position may be modified by thecomputer 414, which sends the appropriate command to the low-level motorcontroller 412 via the step-direction method. Step responses from thehighest stiffness (100%) to several stiffnesses are shown in FIG. 5.Specifically, FIG. 5 depicts a graph plotting step responses fromstanding stiffness (100%) to other stiffnesses. The dashed line in FIG.5 denotes the step response to the slider position of the primarystiffness (47%). The maximum speed of the slider is 80 mm/s, and islimited by the motor and gearhead's maximum permissible speed. Desiredposition, actual position, and current were recorded by the EPOS2 Studiosoftware (Maxon Motors, Switzerland).

Frame. In addition to impact forces and the weight of the person usingthe prosthesis 201, there can be large forces in the frame 262 from thehigh loads on the leaf spring 210. The frame 262 may therefore beconstructed from a material of high strength, high stiffness, and lowweight, such as 7075-T6 Aluminum. The ankle joint 205 may havemechanical hard stops, for instance at 30° of plantarflexion and 30°dorsiflexion. Angular contact bearings can support the cam profile 252.The prosthesis 201 may be attached to a socket worn by an amputee via anadaptor 256, such as a titanium pyramid adaptor with rotationaladjustability (Bulldog Tools, Inc., Lewisburg, Ohio, USA).

Cam Design—Mathematical Modeling.

In order to obtain the primary stiffness curve for the ankle prosthesis201, an appropriate cam profile 252 may be designed. A summary of anexemplary strategy for derivation of the cam profile 252 can besummarized as follows, and is shown in FIG. 8. In 801, the leaf spring210 may be modeled as a rotary spring centered at a simple support. In802, the problem may be framed using the principle of virtual work,where the energy stored in the ankle prosthesis 201 is equal to theenergy stored in the spring. In 803, solve for rotational deflection ofthe spring. In 804, use the geometry to determine cam radius from ankleangle and spring angle. In 805, convert from ankle space to camcoordinates.

FIG. 3A and FIG. 3B display diagrams that reflect a mathematicaldescription of the cam profile and the cam follower. The cam follower issimplified to a point (indicated by the triangle 301). Downwarddeflection of the cam follower (γ) is resisted by the leaf spring(M_(S)), which has a ‘virtual pivot’ located at the simplesupport/slider, and is considered a rotational spring. Deflection of theankle (θ) causes a moment at the ankle (M_(A)), produced by the forcebetween the cam follower and cam profile. The leaf spring is preloaded asmall angle γ0 (the unpreloaded position is shown). FIG. 3B reflects thegeometry used to find the cam profile in polar coordinates (r, ω) from θand γ. The variables d and σ (sigma) are known constants related to thegeometry of the ankle prosthesis. The problem can be simplified in thatthe cam follower may be modeled as a point, as mentioned above. Thefinal cam profile may require a perpendicular offset equal to the radiusof the cam follower.

It cannot be assumed that a spring such as a fiberglass leaf springbehaves like a linear, translational spring. In the example of theprosthesis 201 shown in FIGS. 2A and 2B, the cam follower 254 isattached above the neutral plane of the spring 210, so the horizontalcomponent of the force on the cam 250 creates a moment on the spring210. Additionally, the cam follower 254 does not travel straight down asthe leaf spring 210 deflects, but follows an arc about a virtual pivot.This virtual pivot point may be modeled as being at the simple support.Alternatively, a more accurate model of the virtual pivot point may bepolycentric, dependent on the angle of the load, and somewhere betweenthe simple support and the cam follower. Another source of seriescompliance is the flexure of the aluminum frame. Thus, withoutcorrection, the torque-angle curve may be less stiff than intended.Series compliance at the ankle joint may be modeled at the ankle joint,so that the moment on this compliance is equal to the moment on theankle. The associated deflection is depicted in the figures as S. Forexample, the series compliance may be 1200 Nm/rad.

In an example, the cam profile may be determined by using a mathematicalapproach based on the principle of virtual work. The energy stored inthe ankle joint is equal to the sum of the energy stored in the physicalspring and the unwanted series stiffness, as reflected in Equation 1below:

∫₀ ^(y) M _(S) dγ=∫ ₀ ^(θ) M _(A) dθ−∫ ₀ ^(δ) M _(A) dδ.  (1)

The plantarflexion and dorsiflexion regions may be individually solved,so the lower limit of integration is at the equilibrium position. Theankle moment M_(A) is defined as a function of θ, and the seriesstiffness experiences the same moment, so its associated deflection isreflected in Equation 2 below:

$\begin{matrix}{{\delta = \frac{M_{A}}{k_{2}}},} & (2)\end{matrix}$

At the equilibrium position θ=0°, the spring is preloaded a small angleγ₀. Equation 1 can be written as Equation 3, below:

∫₀ ^(γ) k(γ+γ₀)dγ=∫ ₀ ^(θ) M _(A) dθ−∫ ₀ ^(δ) M _(A) dδ.  (3)

In Equation 3, k is the stiffness of the leaf spring (as a rotaryspring). The left side of this equation may be integrated to:

$\begin{matrix}{{{\frac{1}{2}k\; \gamma^{2}} + {k\; \gamma_{0}\gamma} + c} = {{\int_{0}^{\theta}{M_{A}d\; \theta}} - {\int_{0}^{\delta}{M_{A}d\; {\delta.}}}}} & (4)\end{matrix}$

where c, the constant of integration, can be found to equal zero fromthe initial conditions: θ=0, γ=0, δ=0. We then solve for γ as a functionof θ with the quadratic formula, so that

$\begin{matrix}{{\gamma (\theta)} = {{- \gamma_{0}} + \sqrt{\gamma_{0}^{2} + {\frac{2}{k}\left( {{\int_{0}^{\theta}{M_{A}d\; \theta}} - {\int_{0}^{\delta}{M_{A}d\; \delta}}} \right)}}}} & (5)\end{matrix}$

The integrals in Equation 5, which represent the work stored by theankle and the series compliance, can be numerically solved, since we arespecifying torque-angle relationship at the ankle. From γ, we can deriver, the radius of the cam profile at a given θ, from the defined geometry(via law of cosines) as:

r(θ)=√{square root over (l ² +d ²−2ld cos(γ+σ))}  (6)

where σ is the angle between the initial position of the spring and d, aline drawn between virtual spring centers. Since the cam follower doesnot travel only vertically (see FIGS. 3A and 3B), we need to find ψ, inorder to have a polar representation of the cam profile as (ψ, r):

ψ(θ)=θ_(cam)−α=θ−δ−α  (7)

where α is the small angle between r and vertical, due to the leafspring not deflecting straight downward. We can trigonometrically findα, using the geometry of the setup and the law of sines:

α(θ)=sin⁻¹(L sin(σ+γ(θ)))+ω  (8)

where d and y are respectively the diagonal and vertical distancesbetween the ankle axis and virtual pivot of the spring, and σ is theangle between the line d and the initial position of the leaf spring.The numerical results, now in polar coordinates as (r, ψ) from Equations6 and 7, are converted to Cartesian coordinates, and an offset curvecorresponding to the final cam profile is created with a perpendicularoffset of the cam follower radius.

FIG. 12 displays an enlarged view of the cam profile 252 with an outeredge that is shaped in accordance with the methods described above. Theouter edge has a curved shape is comprised of a first portion 252 a, asecond portion 252 b, and a third portion 252 c. The first portion 252 ais the portion against which the cam follower 254 rolls duringplantarflexion of the prosthesis 201. The second portion 252 b is theportion in which the cam follower 254 is positioned during equilibrium(for instance, as shown in FIG. 2B). The third portion 252 c is theportion against which the cam follower 254 rolls during dorsiflexion ofthe prosthesis 201. As shown in FIG. 12, each section is curved, withsecond portion 252 b having a concave shape with reference to the camfollower 254, and first portion 252 a and third portion 252 c eachhaving a concave shape with reference to the cam follower 254.

Choosing a Cam Profile.

In an example, the cam profile 252 may be optimized for a 70 kg subjectwalking with the slider 220 positioned at 47% of the range of slidermovement. According to the forward model, this position allowed anappropriate range of stiffnesses, higher and lower than what is neededfor walking. The predicted range of stiffnesses and the desired primarystiffness curve for walking are plotted in FIG. 6. As shown in FIG. 6, aprimary desired quasi-stiffness curve for walking is depicted in adashed line, and the range of possible stiffnesses predicted for therange of positions of the slider 220 are indicated in the shaded aresurrounding the dashed line.

Weight.

In an example, the ankle, not including electronics, weighs 908 g. Theelectronics, which can be mounted more proximally on the prostheticsocket to reduce effects on the dynamics of leg swing, may weigh 170 g(including battery).

Stiffness Modulation.

To minimize weight and size, the stiffness modulating mechanism wasdesigned to only be utilized when no load is applied to the spring(other than a slight preload). FIG. 4A illustrates how moving the simplesupport towards the load increases spring stiffness. Experimentalresults of translational leaf spring stiffness is shown as a function ofthe slider position. Individual points in the graph in FIG. 4B representtested positions. As shown in FIGS. 4A and 4B, as the slider positionmoves away from the beam support, the beam stiffness increases as shown,for example, in the graph shown in FIG. 4B.

The torque-angle relationship may be measured using a custom rotationaldynamometer and the encoder 258 on the ankle joint 205. In an example,the dynamometer comprises a motor (BSM90N-3150AF, Baldor, Fort Smith,Ark.) and 6-axis load cell on the motor output (45E15A M63J, JR3, Inc.,Woodland, Calif.). The ankle encoder may be sampled at 100 Hz by theonboard computer, and the load cell may be sampled at 1 kHz. The anklemay be fixed to a pyramid adapter on the dynamometer, and the ankle axismay be aligned with the axis of the dynamometer. The dynamometer canmove the ankle at a constant speed in either plantarflexion ordorsiflexion up to and back from the peak angle over a period of eightseconds. The peak angle tested can be smaller at stiffer settings, so asnot to overload the spring, cam follower, or frame. The test may berepeated at 10 slider positions across the slider's range of motion.

An experimental characterization of the stiffness profiles is shown inFIG. 7. The nine solid lines reflect testing on nine different sliderpositions across the range of motion of the slider 220. Dashed line 705reflects the desired primary stiffness profile, which was designed for aslider position of 47%. Dashed line 710 reflects the experimentallydetermined profile at that slider position. Stiffness around 0°(measured as the average stiffness between −1° and 1°) at the higheststiffness level is 9.2 Nm/deg, and for the lowest stiffness level is0.68 Nm/deg. Averaging across dorsiflexion trials, hysteresis resultedin a loss of only 1.9% of the energy stored.

The specific primary stiffness curve and primary slider position may beselected to balance several design features that may enhance clinicalimpact. A higher stiffness can improve standing stability, and a lowerstiffness to improve range of motion during stair and ramp traversal.The selection of primary slider position governs how the stiffnessprofile can be varied from the primary stiffness profile. Thus, byselecting the primary slider position to be offset to either theanterior or posterior range of the spring, the available stiffnessprofiles can be shifted towards a more stiff or less stiff range,respectively. Thus, the primary stiffness profile and the selection ofthe primary slider position enables further customization of themechanics, which may be altered depending on the design goals.

In an example, stiffness of the ankle can be modulated only during theswing phase of gait, or during static standing, when the ankle hasnegligible torque. By accepting this limitation, the sliding mechanismin the ankle can be built with no bearings or linear rails, because ofthe very low dynamic loads. The interface between the slider and framemay be comprised of two 1 mm thick pads of a high load thermoplastic,such as Delrin® (DuPoint USA, Wilmington, Del.), and the rolling contactwith the spring rotates in high-load bushings. In particular, linearrails are significantly heavier, and the high static loads wouldprohibit the use of rails that could fit in the desired anatomicalenvelope. If the spring is compressed (by either a plantar- ordorsiflexion torque), friction between the roller and slider, or betweenthe slider and Delrin pad, is too great for the motor to overcome.

Combining Intent Recognition.

Adjusting ankle stiffness continuously during gait transitions may beachieved when using an intent recognition system, for instance, the kinddescribed in U.S. Pat. No. 9,443,203 by Aaron Young and Levi Hargrove,titled Ambulation prediction controller for lower limb assistive device,incorporated herein by reference in its entirety. The computer 414 mayexecute the computer instructions used to recognize the intendedambulatory mode of the prosthesis 201, such as standing, walking, ortraversing stairs. In other examples, a change in ambulation mode may bedetected more simply, for instance using a decision table.

Advantages.

The means of modulating stiffness described herein have severaladvantages. The range of stiffness profiles able to be implementedvaries by an order of magnitude, due to the increasing rate of stiffnessvariation as the slider 220 moves posteriorly towards the heel 265. Thisnonlinearly increasing stiffness, coupled with the low inertia forslider 220, leads to a high stiffness modulation bandwidth.Additionally, by moving the slider 220 with a lead screw 232, which isnon-backdriveable, the DC motor 230 is not required to maintain aholding torque, thus decreasing the electrical energy required for useof the prosthesis 201. Finally, the prosthesis 201 can be convenientlypackaged in the anatomical envelope of the human foot-ankle complex,maintaining a cosmetic and low profile for amputees with longer residuallimbs.

The simple mechanics of the prosthesis 201 improves robustness andavoids the more expensive actuation and transmission components thatgenerally come with fully powered prostheses which add or dissipateenergy. The prosthesis 201 is generally lighter than heavier fullypowered prosthesis.

In another example, an ankle prosthesis is fully passive, therebyenabling the prosthesis to require no additional, outside power sourceor computing. This exemplary prosthesis may be made less expensively andmay be used by a broader population. This example ankle prosthesis doesnot need to charge a battery. Additionally, it does not require anintent recognition algorithm that adjusts the ankle's mechanicsautomatically. The ankle prosthesis may have mechanics that are manuallyadjustable, for instance, by the user of the prosthesis. For example,the ankle prosthesis mechanics may be adjusted using a lever connectedto a Bowden cable, which is a type of flexible cable used to transmitmechanical force by the movement of an inner cable relative to a hollowouter cable housing. This would allow the wearer to adjust theprosthesis mechanics to a setting (e.g. “stair setting”) manually atwill.

FIG. 10 displays a side view of an exemplary ankle prosthesis 101. Theprosthesis 101 employs several of the same designs as in prosthesis 201,including the cam-based transmission, its lightweight aspect, itsability to fit into an anthropomorphic physical envelope, and the designof the cam profile.

The prosthesis 101 comprises a lever transmission that permits themodification of the slider position by hand, circumventing the need forthe small electric motor to adjust the position of the slider 120. Asshown in FIG. 10, the lever transmission comprises a lever 130, a pulleyblock 131, a cable 132, a cable covering 133, and slider stoppers 166.Adjustment of the position of the lever 130 moves the slider along andbeneath the leaf spring 110, which in turn, changes the ankle mechanics.At specific positions of the lever 130, the foot mechanics will changeappropriately for different mobility tasks. For example, the levertransmission may include a lever setting for each task, includingwalking, standing, stair ascent/descent, and ramp ascent/descent. Thelever 130 may be mounted anywhere for ease of access. For example, thelever 130 may be mounted on the ankle prosthesis 101, on the prostheticpylon, on the socket, or on another location that the user can quicklyand easily access throughout the day. The lever transmission enablesmovement of the slider, thereby allowing the user to change the anklemechanics. For example, adjusting the position of the lever 130 causesthe cable 132 to move around the pulley block 131. The slider 120 isaffixed to a position on the cable 132 so that movement of the cable 132causes the slider 120 to move along the length of the cable 132 betweendistal stopper 166 and proximal stopper 167.

In an exemplary prosthesis, the lever may comprise a Bowden cabletransmission that adjusts the slider position. A Bowden cabletransmission, which is similar to a bicycle cable, may be preferred dueto the flexibility of lever location, simplicity, and robustness. Thespecific lever and Bowden cable implementation can be configured basedon the distance needed to travel (about 6 cm), and the desiredrotational range of the lever. The diameter of the cable can be minimal,since there is very little friction between the slider and theprosthesis frame due to the Teflon pads.

The use of a leaf spring system to assist in gait may be incorporatedinto other assistive devices, such as orthoses. For instance, anorthosis 500 is shown in FIG. 11 that provides support to a leg 530. Theorthosis 500 comprises an upper brace 502, and a lower brace 518 coupledby a pivotable connector 520 that serves as an artificial ankle joint ofthe orthosis 500. A support member 506 may be attached to the upperbrace 502 and a leaf spring 504. A motor 508 drives a screw 510, and aslider 512 comprising an outer portion 512 a and an inner portion 512 bis adjustable along at least a portion of the length of the screw 510.The leaf spring 504 is attached at its end with an encasement 514,encasing a cam follower 516 that rolls along a profile 517 while theorthosis 500 is in use during gait of the leg 530. The cam transmissioncomprising the cam follower 516 and the profile 517 enables acustomizable nonlinear torque-angle curve. The shape of the profile 517may be determined by using a mathematical approach based on theprinciple of virtual work as described above. As shown in FIG. 11, theshape of the profile 517 is curved and includes a concave portion inwhich the cam follower 516 is positioned while the orthosis is in anequilibrium position, and convex portions along which the cam follower516 rolls while the orthosis 500 is assisting with plantarflexion anddorsiflexion. The slider 512 provides sliding support to shift thiscurve to be more or less stiff overall. In other examples, the camtransmission could be housed on the anterior, lateral, or posterior sideof the shank of the orthosis. Integration of the cam transmission intoan orthosis such as the orthosis 500 can improve gait and a range ofother mobility tasks for a broad spectrum of patients using orthoses,such as stroke, spina bifuda, multiple sclerosis, and incomplete spinalcord injury.

What is claimed is:
 1. A cam system for an assistive device comprising:a. a cam profile and a cam follower, the cam profile having a curvedouter edge comprising a concave portion, the cam follower beingpositioned within the concave portion when the assistive device is in anequilibrium position.
 2. The cam system of claim 1, wherein the camprofile is positioned to rotate about a joint of the assistive device inresponse to a force applied to the assistive device during a stancephase of gait.
 3. The cam system of claim 1, wherein the cam follower iscoupled to a leaf spring of the assistive device.
 4. The cam system ofclaim 1, wherein the cam follower rolls along the curved outer edge ofthe cam profile from dorsiflexion to plantarflexion of the assistivedevice.
 5. The cam system of claim 1, wherein the cam profile has aconvex portion on which the cam follower is positioned duringplantarflexion of the assistive device.
 6. The cam system of claim 1,wherein the cam profile has a convex portion on which the cam followeris positioned during dorsiflexion of the assistive device.
 7. Anassistive device comprising the cam system of claim 1, furthercomprising a leaf spring.
 8. The assistive device of claim 7, whereinthe cam system is positioned at an end of the leaf spring such that aforce applied to the cam system causes deflection of the leaf spring. 9.The assistive device of claim 7, further comprising a sliding element toadjust the stiffness of the leaf spring upon deflection.
 10. Theassistive device of claim 9, wherein the position of the sliding elementis adjustable along a surface of the leaf spring.
 11. The assistivedevice of claim 10, wherein the sliding element is positioned on amotor-powered screw that adjusts the position of the sliding element onthe surface of the leaf spring.
 12. The assistive device of claim 10,wherein the position of the sliding element along the surface of theleaf spring is manually adjustable.
 13. The assistive device of claim12, further comprising a Bowden cable attached to the sliding element tomanually adjust the position of the sliding element.
 14. The assistivedevice of claim 7, wherein the assistive device is an ankle prosthesis.15. The assistive device of claim 7, wherein the assistive device is anorthosis.